1. Field of Invention
The present invention is directed to a method for making photoluminescent water soluble, non-toxic metal, semiconductor nanocrystals that enable biological imaging and analyte detection using near-infrared emissions.
2. Brief Description of the Prior Art
Semiconductor nanocrystals, hereinafter referred to as quantum dots (QDs), with surface bioconjugates have been studied extensively because of their unique optical properties. QDs are inorganic nanoparticles that emit light at a specific wavelength when excited. When light impinges on the QDs, electrons in the valence band are excited to the conduction band, forming short-lived (nanoseconds) electron-hole pairs called excitons that emit photons of a specific wavelength when the electron-hole pairs eventually recombine. The excitonic emission is not as dependent on the excitation light wavelength as that of fluorescent molecules. Therefore it is easier to excite QDs to luminescence than to excite traditional fluorescent molecules that require a specific excitation wavelength. The wavelength of the emitted photons of QDs, however, is specific to and controlled by the composition of the QDs and defect states inside the energy gap.
In the last few years, there has been is an interest in using QDs in biomedical imaging due to advances in surface modification of QDs that have made them accessible for antibody immobilization and detection of antibody-antigen binding. Recent advances enable QDs to be used as imaging markers inside living organisms and as biological markers to find a disease as well as to carry a drug to the exact cell that needs it by immobilizing antibodies on the surface of the QDs. QDs may also be specific to a particular disease and may be tailored to bind only to infected cells. Detection may be carried out either by locating the QDs' particles or by detecting signals emanating from the QDs' particles. For example, luminescence of antibody-coated QDs bound to the cancerous tissue in a mouse helped to locate a tumor (Quantum Dots Get Wet, Science, volume 300, p. 80, Apr. 4, 2003). Until now the main biological tags that have been employed are organic fluorophores or radioactive labels (S. G. Penn, L. He, and M. J. Natan, “Nanoparticles for Bioanalysis”, Curr. Opin. Chem. Bio., 7, 1-7, (2003)).
The fabrication process of water-soluble luminescent QDs, however, is prohibitively expensive and complex, typically requiring the elimination of QD broadband emissions, thus compromising the commercializability of the QDs.
Additionally, the conventional QDs do not enable near infrared imaging (NIR). Consequently, autofluorescence becomes a challenge for tissue imaging due to spectral overlap. Two major sources of autofluroescence in tissue are elastin and collagen. The fluorescence maxima are in the spectral range between 405 and 460 nm with the excitation source between 270 and 370 nm. It is desirable to have emissions at higher wavelengths for medical imaging in order to reduce or eliminate interference from the autofluorescent signals of tissue.
Conventional QDs also utilize toxic heavy metal elements that are poisonous to the human body, rendering them unsuitable for various bioconjugation and biological applications. For example, Qdot 705™ and Qdot 800™ from Invitrogen made from CdSeTe contain toxic elements.
In view of the aforementioned deficiencies, there is a need to develop enhanced non-toxic highly luminescent QDs capable of producing near-infrared emissions without expressing undesirable broadband emissions.